Most commercial properties include a number of building systems that monitor and regulate various functions of the building for the comfort and well-being of the tenants. These building systems include security systems, fire control systems, and elevator systems. One prevalent and important building system is the environmental control system.
An environmental control system is used to regulate the temperature and flow of air throughout a building. The air conditioning for a building typically includes one or more chillers for cooling air and one or more heaters for warming air. Fans distribute air conditioned by a chiller or heater into a duct system that directs the flow of air to the various rooms of a building. Dampers controlled by terminal units are located within the duct system to variably control an opening to a branch of a duct system or to a room. The dampers are maneuvered through a range of movement from being 100% open to 0% open, i.e., closed, by actuators. Also, the speed of a motor that drives a fan is controlled to regulate fan speed and, correspondingly, air flow in the system. An important component of a building environmental system is the control system that varies the fan motor speed and the position of the various dampers to maintain pressure and flow rate set points for the system.
Control systems for building systems are increasingly reliant upon computer and network technology. Micro-controllers and the like may be used to operate and maintain actuators for damper position as well as controlling fan speed. These local controllers provide operational data to an overall system controller. The overall system controller is typically a computer that analyzes data received from local controllers to determine whether system parameters, such as set points, are being maintained. If the system parameters are not being met, the system controller issues command messages to one or more local controllers to adjust local control so the system parameters may be achieved. In some previously known systems, the system controller communicates with the local controllers over a computer network. Most typically, the hardware platform for the network is an Ethernet platform and the network software supporting communication over the network is a TCP/IP layer. This structure supports communication between a control application program executing on the system controller and an application program executing on the local controller.
In order to monitor a building system, a system controller typically receives status data from sensors, such as flow sensors, coupled to local controllers or terminal units. These data may be compared by the system controller to the system parameters or set points to determine the control actions required to bring the system into compliance with the system set points. One type of environmental system is a variable air volume (VAV) system. In an exemplary VAV system, such as the one shown in FIG. 1, outside air is drawn into system 10 through an outside damper 14, filtered through filter 18, and warmed, if necessary, by heating coil 20. A single or dual fan 24 pushes the air through cooling coil 28 for cooling, if required, and filtered again by filter 30 before being supplied to terminal units 32a, 32b, 32c, 32d, and 32e for distribution through diffusers 34 to zones serviced by the terminal units. Terminal units 32a–32e typically supply air at a constant temperature of 15 degrees Celsius for cooling and 22 degrees Celsius for heating. A fan controller regulates the speed of fan 24 to generate adequate pressure to overcome the resistance provided by the coils, filters, air ducts, and dampers. Typically, the pressure set point used by the fan controller to regulate the fan speed corresponds to an arbitrary location that is generally two-thirds of the distance to the terminal unit that is farthest downstream from the fan. This pressure may be sensed by a static pressure sensor 38. The pressure sensed at this location varies as the flows to the various zones change in response to thermal requirements detected for a zone. The fan controller uses the sensed static pressure to either modulate the fan speed or adjust the position of the outside damper to maintain the fixed pressure set point.
Consumption of electrical energy by fans in heating, cooling, and ventilation (HVAC) systems is significant. According to a 1999 Department of Energy report, central VAV fans in commercial buildings in the United States consume 62.7 billion kWh. If this electrical energy usage could be reduced, not only would the operators of the building systems see a financial savings but the amount of pollutants generated during the production of electrical energy would be lowered as well. Thus, there is a private and a public benefit in reducing the amount of electrical energy consumed by the operation of building system, such as a HVAC system.
One way to reduce building HVAC electrical power consumption is by delivering airflow at a fraction of maximum capacity when a lower airflow rate is required to satisfy only a fraction of the maximum space cooling or heating energy demand. This is usually achieved by modulating the fan speed of a central air handler. Centrifugal fans are the most common type of fan used in central HVAC systems. These fans consume energy in proportion to the product of the airflow rate and fan pressure. Therefore, lowering the airflow rate in this type of HVAC system, commonly called a variable air volume (VAV) system, also reduces electrical energy consumption of the system. Typically, the pressure set point for the arbitrary location located downstream from the fan is usually selected so that the fan is able to supply maximum air flow to all of the zones when they are experiencing system design conditions. However, the zones regulated by the terminal units rarely experience the design conditions. Consequently, the zones typically require only a fraction of the design condition and this means that the pressure set point is significantly higher than what is required for operation of the building system most of the time. The chapter entitled Automatic Control in the ASHRAE Handbook on HVAC Applications, 1995, states that excessive duct static pressure leads to poor system control, noise, and waste of electrical energy. Hence, there is a need to more effectively determine the pressure set point for controlling the operation of a VAV HVAC system.
The process of installing the components of a HVAC system and initially determining the operational set points is known as commissioning a HVAC system. The ducts, terminal unit, and diffusers that service a zone are sometimes called a branch. A fan typically supplies air to more than one branch. The control signal required for proper regulation of a damper does not necessarily correspond to the expected air flow through a terminal unit but rather the actual air flow through a branch. Thus, commissioning requires measurements of air flows through branches under differing conditions so actual air flows may be used to determine expected air flows for control purposes. The measurements are used to compute flow loss coefficients that correlate the manual flow measurements to the flows measured by a flow sensor located near a damper. The flow loss coefficient is manually entered into the terminal unit so the local controller properly regulates air flow to the zone serviced by the branch. The process of measuring air flows and computing the flow loss coefficient is repeated for each branch supplied by a fan. If there are errors made during the process of computing the initial flow loss coefficients or system configuration changes made, the process must be repeated for each branch. Furthermore, as the system ages, flow loss coefficients for a branch may change without detection. Only during re-commissioning of the system are such changes detected.
To address the need for simplifying the procedure for computing flow loss coefficients, a system was developed for performing a self-commissioning process. This system is described in U.S. Pat. No. 5,705,734 and is commonly owned by the assignee of this patent. The disclosure of the '734 Patent is hereby expressly incorporated by reference. The procedure of the '734 Patent requires a determination of the main supply duct segment flow loss coefficient by measuring fan flow rate at two different terminal unit conditions while holding flow rate through one of the terminal units at a constant rate. Using the flow loss coefficient for the fan supply duct segment and measuring flow and pressure conditions for other terminal unit conditions, the flow loss coefficients for the remaining main duct segments may be computed. Once the flow loss coefficients for the main duct segments are computed, the flow loss coefficient for a terminal unit may be determined by closing all other terminal units and determining the flow through the open terminal unit from the main duct segment flows. This procedure is repeated for each terminal unit. While the method and system of the '734 Patent simplifies the data collection for flow loss coefficient computation, it does rely upon closure of the terminal units other than the one for which the flow loss coefficient is being computed. Terminal unit closure assumes no leakage of air through a closed terminal unit; however, such an assumption is rarely accurate. Also the system and method of the '734 Patent implements a sequential process for determining the flow loss coefficients of duct segments and terminal units.
One proposed solution for determining a fan static pressure set point suggests the use of flow set points for terminal units in an existing system with software for designing duct systems. However, this proposal does not address how existing duct structure information may be collected for use in the solution and duct design software is not conducive for real time applications. Another proposed solution assumes all branches of a system have the same airflow rate and this solution works well for small structures, such as most houses. However, application of that approach to commercial properties does not appear feasible because large systems include multiple main duct segments and each one may have different airflow rates. Another problem with this solution is that it requires a low flow rate through one open terminal while all other terminal units remain closed. Such a condition is difficult to maintain in large building systems.
Another proposed solution requires generation of error signals from the terminal units that are either provided to a proportional integral (PI) controller or a heuristic analysis of the error signals and their changes. However, the handling of the error signals in those systems presented significant issues. Other systems use a trial and error approach of gradually lowering the fan pressure by a fixed amount until a terminal unit asserts a low flow rate alarm condition. Establishing the fan set point at a level that results in a flow rate at one or more terminal units that is barely above its alarm level may inadequately serve a room in a typical commercial building. In a research facility where air flow rate may also be important for safety reasons, such an operating condition is even more likely to be unacceptable. Solutions that rely upon damper position signals rather than pressure or flow rate signals also present issues. In this type of system, the fan rate is gradually increased until one of the terminal units is almost fully opened. However, damper position sensors increase implementation costs and real time position measurements require complex signal processing and data analysis, especially with transient data.
What is needed is a system and method for determining flow loss coefficients and set points for a building system that do not significantly increase implementation costs of HVAC systems.
What is needed is a system and method for determining flow loss coefficients and set points for a building system that do not result in marginally acceptable air flow rates.
What is needed is a system and method for determining flow loss coefficients and set points for a building system that do not require the generation and processing of error signals.
What is needed is a system and method for determining flow loss coefficients and set points for a building system that do not require complex software descriptions of building duct systems.
What is needed is a system and method for determining flow loss coefficients and set points for a building system that is applicable to large building systems.